1. Field of the Invention
The present invention relates to an optical system which is provided with an image stabilizing function for correcting image blurring arising from the movement of the optical system. More particularly, the present invention relates to an optical system with an image stabilizing function which employs an inner focus method in which a movable lens unit for image stabilizing is moved in a direction perpendicular to the optical axis of the system to preclude a drop in optical performance.
2. Description of the Related Art
If picture taking is attempted in a moving vehicle such as a car or an airplane in motion, an imaging system (an imaging lens) is subject to shake, thereby blurring images.
When an imaging system having a long focal length is used, the controlling of shake of the imaging system is particularly difficult. If the imaging system is tilted due to shaking, the resulting image is displaced according to the tilt angle and the focal length of the system. For this reason, a sufficiently long photographic operation time is required to prevent image degradation in a still picture taking device, and the maintaining of a photographic composition is difficult in a moving picture taking device. When the imaging system is tilted due to shaking during picture taking, it is necessary to correct image blur.
Image stabilizing optical systems for precluding image blurring are disclosed, for example, in Japanese Patents Laid-Open Nos. 50-80147 and 56-223819 and Japanese Examined Patent Publication 56-21133.
Japanese Patent Laid-Open No. 50-80147 discloses a zoom lens having two a focal variable power systems, namely a first variable power system having an angular magnification of M1 and a second variable power system having an angular magnification of M2. Both variable power systems vary respective magnifications to maintain the relationship of M1=1xe2x88x921/M2 while the second variable power system is spatially secured to correct image blurring and thus to stabilize images.
According to Japanese Examined Patent Publication No. 56-21133, part of an optical member in an optical device is moved in the direction of stabilizing an image to cancel out the movement of the optical device in response to an output signal from detector means for detecting the movement of the optical device.
Japanese Patent Laid-Open No. 61-223819 discloses an imaging system having a variable apex angle prism at its side closest to an object. The apex angle of the prism is varied in accordance with the movement of the imaging system to deflect an image for image stabilization.
The inventor of this invention discloses, in Japanese Patent Laid-Open Nos. 7-168135 and 8-304698, an image stabilizing optical system in which a lens unit in an imaging optical system is moved in a direction perpendicular to the optical axis of the system to compensate for image blurring arising from the movement of the entire system.
Still image taking devices, which preclude image blurring by moving part of a lens unit of its imaging system preferably have the following features: a large quantity of correction for image blurring, a small linear or angular travel of the lens unit (movable lens unit) that is moved for correction of image blurring, and a compact overall size.
When a large decentering aberration takes place with the movable lens unit decentered, the decentering aberration causes a resulting image to blur even when image blurring correction is attempted.
In an image stabilizing optical system, decentering aberration must be small when the movable lens unit is moved in the direction perpendicular to the optical axis for decentering. A small travel of the movable lens must compensate for a large quantity of image blurring; more particularly, a decentering sensitivity (a ratio of unit travel xcex94H to an image blurring correction xcex94x, xcex94x/xcex94H) has to be large.
In the image stabilizing optical system in which an optical member is spatially secured against the movement of the system, the supporting of such an optical member is difficult and further the implementing of compact design in the optical system is also difficult. An optical system having the variable apex angle prism at its side closest to the object has the advantage that practically no aberrations other than decentering color aberration take place. The optical system has disadvantages that a bulky driving member is needed and that decentering color aberration taking place with the prism is not simply corrected. In the image stabilizing optical system in which part of a lens unit in the imaging optical system is decentered, the optical system may be made compact by selecting and arranging the decentered lens unit properly. This system however cannot cover a large correction range with a small decentering driving input while at the same time correcting decentering coma, decentering astigmatism, decentering curvature of field, and the like.
It is an object of the present invention to provide an optical system with an image stabilizing function that is suitable for use in a telephoto imaging optical system of an inner focus type, wherein when part of a lens unit in the optical system is decentered in a direction perpendicular to the optical axis of the system to correct the displacement (blurring) of an image, each decentering aberration is properly corrected by arranging each lens element and the entire system is made compact by allowing a sufficiently small decentering driving input to cover a large displacement correction (image blur correction) range.
(1-1) The image stabilizing optical objective lens system of the present invention comprises, in the order from the side of the system closest to an object, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power, wherein the second lens unit is moved in the direction of the optical axis of the system for focusing and the fourth lens unit is moved in the direction perpendicular to the optical axis for correcting image blur.
(1-2) The image stabilizing optical objective lens system of the present invention comprises, in the order from the side of the system closest to an object, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power, wherein the second lens unit is moved in the direction of the optical axis of the system for focusing and the fourth lens unit is moved in the direction perpendicular to the optical axis for correcting image blur, and wherein when the focal length of the entire system is normalized to 1, the following relationships hold true:
xe2x88x925.0 less than I4 less than xe2x88x920.7xe2x80x83xe2x80x83(1a)
0.7 less than I5 less than 5.0xe2x80x83xe2x80x83(2a)
0.5 less than |I4/I5| less than 2.0xe2x80x83xe2x80x83(3a)
where I4 and I5 represent respectively the spherical aberrations of the fourth and fifth lens units in connection with an object at infinity.